I have had my Nokia N810 for probably 2 years now with the original battery but I didn't use it for a while and the battery still seems to last as long as if it was brand-new.

I have put the OS onto my 8GB micro-SD card using that bootloader I forget the name of just know it looks like Microsoft's XP Bootloader.

Anyway, when i use my USB charger or my other Nokia charger everything works like a charm, I have no problems.

Yet the minute I put in the AC-3U or default charger that came with the device I can have totally random reboots of the device and the other day I awoke to the boot menu with TEST written in large green letters.

I have the same problem. It happens only if battery is below 50%. I have found a thread here, that explains how to find exact reason for this. Mine is the battery. I confirmed it with exchange my battery with the one of my phones. Reboots stopped. Anyway you should find the real reson and proper solution fo your case. Just search TMO.